Philip Bennetts QC, prosecuting, told the court Fairweather had made a "freely formed decision" to carry out the killings.

Mr Bennetts said: "He knows in detail what he did, and he is setting about putting forward a defence which he hopes will result in him being found not guilty of murder.

"The prosecution case is that the self reported hallucinations are a fabrication, a fabrication to hoodwink you to acquit him of murder."

The court heard the teenager had twisted rape fantasies included "handcuffing and tying up women who are screaming and saying stop", and beating women "in the face with his fists and forcing sex on them from behind."

ESSEX POLICE

CCTV: The last movements of Nahid Almanea

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'The Moors Murderers', Ian Brady and Myra Hindley murdered five children between 1963 and 1965, at least four of whom were sexually assaulted

Fairweather kept press cuttings about the killings in a plastic folder, and shortly after his arrest last year, he told a psychiatrist "he believed he could have gone on to kill at least 15 more people".

In police interviews he said voices took "control" over his body during the killing of Mr Attfield.

Simon Spence QC, defending, said the youth was "a 15-year-old caught up in what I want to describe as the perfect storm of his autism, increasing isolation and paranoia, leading to a psychosis which led him to kill".

Mr Spence added: "The question for you is what caused that passive interest to become what turned out to be a dangerous obsession that led him to kill twice?

"The only sensible answer we suggest is in this case, for whatever reason, he lost his ability to form a rational judgement or exercise his self control."